Where the Germanic tribes left their heartland to begin their conquest of Europe about the turn of the millennium, their word for YES was ja. Most Germanic languages have retained that form. However, the Anglo-Saxons changed the Old Teutonic ja to gia which became YEA when Anglo-Saxon became English. Anglo-Saxon, a highly inflected language had another affirmative word, gise, a combination of gia + si (the third person singular present subjective of beon, "to be"), meaning "Yes, it is so." Gia + si or YEA + si was originally used as an answer to that class of questions which were intrinsically not improbable.
Word Ways: Vol. 15
, Article 19.
Available at: https://digitalcommons.butler.edu/wordways/vol15/iss3/19